Burning rate modifiers for high energy difluoroamino compounds

ABSTRACT

1. IN LIQUID ROCKET ENGINES UTILIZING AS A PROPELLANT COMPONENT A DIFLUOROAMINO DERIVATIVE OF THE FORMULA   F2N-C(-A)(-Z)-C(-X)(-Y)-NF2   WHEREIN A, Z, X, AND Y ARE EACH A MEMBER SELECTED FROM THE GROUP CONSISTING OF HYDROGEN AND LOWER ALKYL GROUPS OF UP TO FOUR CARBON ATOMS, THE TOTAL NUMBER OF CARBON ATOMS IN A, Z, X, AND Y NOT TO EXCEED SIX, THE IMPROVEMENT WHICH COMPRISES INCORPORATING IN SAID DIFLUOROAMINO DERIVATIVES AS A BURNING RATE MODIFIER A SUFFICIENT AMOUNT OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF FERRIC CHLORIDE, ZINC CHLORIDE, AND STANNOUS CHLORIDE TO FORM A SATURATED SOLUTION THEREOF.

3,582 413 BURNING RATE MODIFIIiIRS FOR HIGH ENERGY DIFLUOROAMINO COMPOUNDS Barry D. Allan and Joseph W. Connaughton, Huntsville,

Ala., assignors to the United States of America as represented by the Secretary of the Army N Drawing. Filed Feb. 15, 1963, Ser. No. 259,485

Int. Cl. C06d /00 US. Cl. 149-109 3 Claims The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to burning rate modifiers for high energy compounds. More particularly, this invention relates to the use of ferric chloride, zinc chloride, or stannous chloride as a burning rate modifier for high energy compounds characterized by the presence of one or more difluoroamino substituents, NF

In the area of liquid rocket propellants, it has recently been determined that high energy compounds containing one or more difluoroamino substituents are excellent fuels which provide substantial increases in rocket motor thrust. One problem associated with the use of these difiuoroamino substituted compounds is their tendency to detonate because of their unusually fast burning rates. It has now been found that these burning rates can be markedly decreased by adding certain burning rate modifiers to the fuels. These modifiers are lead acetate, ferric chloride, zinc chloride, and stannous chloride. Of these burning rate modifiers, ferric chloride is the preferred member. As should be apparent to one skilled in the art, these modifiers are all members of the Lewis acid class of chemical compounds. No doubt, further experimentation with other members of the Lewis acids will show that additional members also function as burning rate modifiers.

In accordance with the foregoing it is an object of this invention to provide burning rate modifiers which improve the burning characteristics of high energy compounds utilized as rocket propellants.

Another object of the present invention is to provide burning rate modifiers which decrease the burning rate of high energy compounds characterized by the presence of one or more difiuoroamino substituents.

The manner in which these and other objects can be accomplished will become apparent from the description given hereinbelow.

The high energy compounds whose burning rates can be modified according to the present invention are any of those containing one or more difluoroamino substituents. However, a preferred group of difluoroamino derivatives are those prepared by the addition of tetrafluorohydrazine to ethylenically unsaturated organic compounds according to the following general reaction An especially preferred group of difluoroamino derivatives are those liquids corresponding to the formula wherein A, Z, X, and Y are each hydrogen or lower alkyl groups of up to four carbon atoms with the further proviso that the total number of carbon atoms in the compound not exceed six. This group is especially preferable due to their excellent performance as rocket fuels in conjunction with their ease of preparation and availability of the raw materials requisite to their manufacture. It is to be understood that the carbon limitation of 3,582,413 Patented June 1, 1971 six is necessitated by the fact that when there are more than six carbon atoms, the compounds are usually viscous or solids and not suitable for liquid fueled rocket engines. However, the burning rate of solids could be modified using the present metal halides.

The examples given hereinbelow are illustrative of the general method of reacting ethylenically unsaturated compounds with tetrafluorohydrazine. The reaction is illustrated with alkenes since the adducts of the alkenes represent the preferred class of difiuoroamino derivatives for the propellant compositions of the invention. However, by substituting for the alkenes other ethylenically unsaturated derivatives such as styrene, allyl alcohol, methyl, acrylate, vinyl acetate and divinyl ether, additional difluoroamino derivatives suitable for use in the liquid monopropellant compositions of the invention can be prepared.

EXAMPLE I Preparation of 1,2-bis(difluoroamino)-ethane There is introduced into a 500 cc. evacuated Pyrex bulb equipped with a stopcock and condensing arm 8.7 l0- mole crude tetrafluoro-hydrazine N F remainder being NF N 0, and NO) and 6.7)(10 mole ethylene. The bulb is then attached to a capillary monometer and heated for fifteen hours at a temperature of C. to C. in an oil bath, During this period of time the pressure in the bulb decreased from 655 mm. to a pressure of 435 mm., both pressures being measured at 110 C. The bulb was removed from the oil bath and the condensing arm immersed in a bath at 196 C. A pressure of about 3 mm. of noncondensable material was pumped from the bulb. The material remaining in the bulb was warmed to room temperature and transferred into the vacuum system. A small quantity of non-volatile material remained in the reaction bulb. The material in the' vacuum system was fractionated through traps maintained at 78 C., 110" C., -l60 C., and 196" C. The fractions at l60 C. and -196 C. were combined and found to represent 4.8 10- moles. The percentage composition of this combined fraction was determined by mass spectral analysis. Eighty-eight percent of the tetrafluorohydrazine had reacted, the ratio of tetrafiuorohydrazine to ethylene consumed in the reaction was 1.16. The NO present as an impurity had reacted while the amounts of N1 and N 0 remained virtually unchanged.

Thereafter, the 78 C. and 110" C. fractions were combined and refractionated for three hours through traps maintained at 46 C., -66 C., and 196 C. The major fraction was found in the 96 C. fraction as a colorless liquid with a vapor pressure of 32 mm. at 0 C. The same pressure was rapidly obtained by warming the sample from 96 C. or cooling the sample from room temperature, indicating the fraction was homogenous. A gas density molecular weight determination on an aliquot of the fraction gave a value of 130.3. The molecular weight of 1,2-bis(ditfuoroamino)ethane would be 132.1. The yield was 55% based on the tetrafluorohydrazine consumed.

Calculated for C H N F (percent): C, 18.19; H, 3.05; N, 21.21; F, 57.55. Found (percent): C, 18.60; H, 3.29; N, 20.55; F, 53.9.

EXAMPLE II Preparation of 1,2-bis(difluoroamino) propane The apparatus was the same as employed in Example I. The reaction bulb was charged with 6.4 10 mole crude N F and 6.1 X 10 propylene. The bulb was heated in an oil bath at 100 C. for eight hours. During this interval, the pressure in the bulb decreased from 518 mm. to 316 mm., both pressures being measured at 106 C. The reaction mixture was fractionated essentially as described in Example I. The major product, collected at 96 C. was a colorless liquid with a reproducible vapor pressure of 21 mm. at C. A gas density molecular weight determination on an aliquot gave a value of 146.9; l,2-bis(difluoroamino)propane requires 146.1. The fraction weighed .45 gram representing an approximate yield of 60% based on the consumption of tetrafluorohydrazine.

Calculated for C H N F (percent): C, 24.66; H, 4.14; N, 19.18; F, 52.02. Found (percent): C, 24.72; H, 4.74; N, 18.05; F, 49.9.

EXAMPLE III Preparation of 1,2-bis(difluoroamino)propane In a 500 cc. stainless steel bomb, a 1.5:1 molar ratio of tetrafluorohydrazine to propylene was heated at 110 C. for two and one-half hours. The pressure decreased during the period of heating from 72 p.s.i. to 21 p.s.i., both pressures being measured at C. The product was distilled through a Holtzman column under an atmosphere of nitrogen yielding about six grams of 1,2-bis(difiuoroamino)-propane which represented a yield of approximately 90% based on the amount of propylene employed in the preparation.

From a comparison of Example II with Example III, it is seen that pressure can greatly facilitat the addition reaction.

EXAMPLE IV Preparation of 1,2-bis (difluoroamino)2-methyl-propane To an evacuated 250 cc. Pyrex bulb was added 1.27 moles of tetrafluorohydrazine and 1 mole of isobutylene. The bulb was heated in an oil bath for six hours at a temperature of 110 C. The product was purified by bulb to bulb distillation which resulted in the recovery of 3.6 g. of the liquid product. This represented approximately an 86% yield. The micro-RP. for the product was 99.5 C. to 100 C. Various reproducible vapor pressure exhibited at different temperatures by the product are as follows: P0 C.=12.1mm.;P15.0 C.=26.1mm.;P27.8 C.=49.3 mm. A quantitative hydrolysis of 61.03 mg. of the product in 35.99 ml. of 0.1111 N NaOH-lO ml. ethanol solution for twenty-one hours gave an equivalent weight of 40.14. The calculated equivalent weight is 40.0.

Calculated for C H N F (percent): F, 47.5. Found (percent): F, 47.2.

By substituting 4-methyl-pentene-1 for the isobutylene of Example IV in the proportions indicated, there is produced 1,2-bis (difluoroamino) 2 methyl pentane. The product is recovered by bulb distillation.

The methods are readily adapted to large scale operation for production of large quantities of material.

Other alkenes such as butene-l and butene-2 can be substituted for the ethylene, propylene, and isobutylene of Examples I-IV to prepare the corresponding tetrafiuorohydrazine adducts.

In order to prepare the saturated solutions contemplated by the invention all that is required is the thorough mixing of the difluoroamino substituted compound with an excess of the particular burning rate modifier desired. The burning rate modifiers are only slightly soluble in the difluoroamino derivatives. Therefore, after mixing the undissolved modifier is filtered from the solution. The following example illustrates the simple mixing procedure.

EXAMPLE V To 1,2-bis-(difluoroamino)-2-methyl-propane is added 1% by weight of ferric chloride. The ingredients are stirred for a period of about 1 minute at ambient tem perature. The undissolved ferric chloride is then filtered from the solution.

Using the standard capillary strand burning tests, the ferric chloride solution exhibited rate decreases of from 10% to 50% of the burning rates of the pure difluoroamino substituted compounds depending on the pressures. Capillaries 8.0 cm. in length and ranging in diameter from .5 mm. to 4.0 mm. were used in the tests. The pressure was varied from 50 p.s.i. to 1000 p.s.i. by enclosing the capillary in a nitrogen pressure bomb. The burning rates were measured by standard photographic techniques.

Ferric chloride dissolved in 1,2-bis(difluoroamino)-2- methyl-propane is illustrative of the invention and data for this solution as found in the above described tests is as follows.

(1)(a) .Using a capillary tube .5 mm. in diameter and 8.0 cm. in length, pure 1,2-bis(difluoroamino)2-methylpropane burned so rapidly that at a pressure slightly less than 900 p.s.i. the burning rate increased to the extent that detonation always occurred.

(1)(b) Employing the same size capillary as above and a saturated solution of ferric chloride in l,2,-bis(difluoroamino)2-methyl-propane, progressive burning without detonation was accomplished up to pressures of 1000 p.s.i., the pressure limit of the test system.

(2) Again using a capillary of .5 mm. diameter and pure 1,2,-bis(difluoroamino)2-methyl-propane at a pressure of p.s.i., a burning rate of 1.3 cm./sec. was measured while under the same conditions the ferric chloride solution had a burning rate of 0.6 cm./sec.

(3) With capillary tubes of 2 mm. in diameter the solution of ferric chloride dissolved in 1,2-bis(difiuoroamino)-2-methyl-propane exhibited a decrease in burning rate of 10% over the pressure range of 100 p.s.i. to 300 p.s.i. when compared to the burning rate of the pure solvent.

Zinc chloride and stannous chloride reduce the burning rate in the same manner although not quite as effectively as ferric chloride.

In addition to preventing detonations, decreasing the burning rate of the difluoroamino substituted compounds allows an increase in the diameter of the injection ports in the injector. The maximum diameter of the injection port possible without having the propellant burn back through the injector ports and fuel lines is commonly referred to as the quench diameter. Thus, the present invention allows larger quench diameters greatly facilitating the injection of the fuel into the combustion chamber of the rocket motor.

This invention can be utilized in rocket engines by employing the saturated solutions of the burning rate modifiers dissolved in .the difluoroamino substituted compounds in lieu of the pure difiuoramino substituted compounds themselves. A particularly useful monopropellant composition comprises a solution of anhydrous nitric acid and 1,2-bis(difiuoramino)12-methyl-propane in a molar ratio of acid to difluoroamino derivative of 2.25:1.0. Another very useful monopropellant composition comprises a solution of dinitrogen tetraoxide and 1,2-bis(difiuoroamino)-2-methyl-propane in a molar ratio of tetroxide to difluoroamino derivative of 1.5:1.0. Substituting the saturated solution of the invention for the pure difluoroamino derivatives in the amounts stated provides useful propellant compositions incorporating the present invention. Since the modifiers are so very slightly soluble in the difluoroamino derivatives, no allowance need be made for the modifiers in preparing the above compositions.

As should be obvious to those skilled in the art, the burning rate modifiers of the invention can be used in bipropellant systems or in systems utilizing a combination of difluoroamino derivatives.

From the standpoint of safety, it is pointed out that the difluoroamino derivatives are quite sensitive to mechanical shock. Therefore, caution should be exercised while handling difluoroamino substituted compounds. By dissolving trace amounts of dinitrogen tetroxide in the difluoroamino substituted compounds, the om ound can be stabilized to shock.

The above detailed description is for purposes of illustration only and no undue limitations should be attributed to the invention because of this description except as reflected in the appended claims.

We claim:

1. In liquid rocket engines utilizing as a propellant component a difluoroamino derivative of the formula wherein A, Z, X, and Y are each a member selected from the group consisting of hydrogen and lower alkyl groups of up to four carbon atoms, the total number of carbon atoms in A, Z, X, and Y not to exceed six, the improvement which comprises incorporating in said difluoroamino derivatives as a burning rate modifier a sufficient amount of a member selected from the group consisting of ferric References Cited UNITED STATES PATENTS 3,347,924 10/1967 Passannante et al. 149-109X BENJAMIN R. PADGETT, Primary Examiner U.S. CL. X.R. 6021 1, 215 

1. IN LIQUID ROCKET ENGINES UTILIZING AS A PROPELLANT COMPONENT A DIFLUOROAMINO DERIVATIVE OF THE FORMULA 